1. Field of the Invention
The present invention relates to linear regulators, and more particularly to a linear regulator that has improved power supply rejection ratio, voltage regulation, power dissipation, and high voltage tolerance and which reduces input filter capacitor requirements.
2. Description of Related Art
Linear regulators are used in many electronic devices and applications for converting an unregulated input voltage to a regulated output voltage. A regulator is intended to hold its output voltage at a design value or within a predetermined voltage range regardless of changes in load current or input voltage. A 3-terminal regulator, which includes an input pin, an output pin and an adjust or ground pin, is a relatively simple and inexpensive realization of a linear regulator typically implemented on a separate chip or integrated circuit (IC). In the typical 3-terminal regulator configuration, a pass transistor is used to control the amount of conduction between the input and output of the regulator based upon a control voltage applied to the pass transistor. An amplifier circuit, such as an operational amplifier or the like, compares the regulator output voltage with a reference signal and adjusts the conduction of the pass device to regulate the output voltage to a predetermined voltage level. Several types of pass transistors can be employed depending upon the desired characteristics of the regulator, such as a PNP bipolar-junction transistor (BJT), or a P-channel metal-oxide semiconductor, field-effect transistor (MOSFET) for low voltage dropout applications, or an NPN Darlington pair driven by a PNP BJT (for standard configurations), or an NPN/PNP BJT pair (for quasi-low dropout voltage applications, etc.
There are several important specifications that exist for any linear regulator configuration. One specification is power supply rejection ratio (PSRR), which refers to the ratio of the change at the output of the regulator relative to the disturbance at the input that caused it. It is desired to have a relatively high PSRR to reduce input voltage supply disturbances as much as possible at the output, particularly input voltage ripple. Another specification is voltage regulation, which refers to the relative change of the output voltage in response to changes in output current or load transients. The output voltage of a regulator may change significantly in response to a significant load or input transient, whereas it is desired to improve regulation and provide greater output stability and regulated voltage accuracy in response to load or input transients. Another specification is the maximum input voltage rating of the regulator. The higher the voltage rating, the greater the applications to which the regulator might be employed thus giving it a larger potential market. An important concern in many applications is the power dissipation in the regulator itself. It is desired to maximize efficiency by minimizing power dissipation in the regulator itself while transferring input power to output power. Dropout voltage input-to-output is an important specification in this circumstance. Dropout is the minimum operating voltage of the regulator input-to-output. Regulators implemented according to low-drop-out (LDO) configurations focus on minimizing drop-out voltage.